The aim of this research project is to explore the regulatory mechanisms involved in cell differentiation and malignant transformation. The model system used consists of clonal derivatives of B16 mouse melanoma, which when grown in culture with or without low concentrations of 5-bromodeoxyuridine (BrdU), become reversibly suppressed in melanogenesis, malignancy, and plasminogen activation. C-type virus production and immunogenicity increase with growth in BrdU, and cellular morphology changes from piled pigmented melanocytes to flat unpigmented fibroblastic cells. Clone C3 471, grown for 1 year in 1 ug/ml of BrdU, is non-tumorigenic and immunogenic, protecting mice pre-injected with these cells against inevitable tumor formation after inoculation of the untreated line (B5 59). B5 59 cells have high plasminogen activator (PA) activity, C3 471 cells none. Mixtures of both cell types result in quenching PA activity of B5 59 cells and such a mixture inoculated into C57BL/6J mice is non-tumorigenic. Cell contact is required for both effects. The mechanism for this correlated suppression is unclear. C3 471 chromosomes display a non-random incorporation of BrdU. Such "hot spots" may provide a morphological clue to regions in the material which regulate the changes in phenotype observed.